The present invention relates to a pixel circuit for current-driving a light-emitting element provided for each pixel. The invention also relates to a display that includes the pixel circuits arranged in a matrix, and particularly to an active-matrix display that controls the amount of a current applied to a light-emitting element, such as an organic electro-luminescence (EL) element, by use of insulated-gate field effect transistors provided in the respective pixel circuits.
In an image display, e.g., a liquid crystal display, plural liquid crystal pixels are arranged in a matrix, and the transmitted intensity or reflected intensity of incident light is controlled for each pixel based on information of images to be displayed, to thereby display the images. The similar principle also holds for an organic EL display employing organic EL elements for its pixels. The organic EL element is a self-emitting element unlike the liquid crystal element. Therefore, the organic EL display has advantages over the liquid crystal display: high image visibility, no backlight, and high response speed. Furthermore, the organic EL display is a current-control display, which can control the luminance level (gray-scale) of each light-emitting element with the amount of a current applied to the emitting element, and therefore is significantly different from the liquid crystal display, which is a voltage-control display.
The organic EL display is driven with a simple-matrix or active-matrix method as with the liquid crystal display. The simple-matrix method employs a simple structure, but involves difficulties of fabricating large-size and high-definition displays. Therefore, the active-matrix displays have been developed more actively in recent years. In the active-matrix method, a current applied to a light-emitting element in each pixel circuit is controlled by an active element (typically a thin film transistor (TFT)) provided in the pixel circuit. Examples of the active-matrix method have been disclosed in Japanese Patent Laid-Opens No. 2003-255856, 2003-271095, 2004-133240, 2004-029791, and 2004-093682.
Conventional pixel circuits are disposed at intersections between row scan lines that supply control pulses and column signal lines that supply video signals. Each pixel circuit includes at least a sampling transistor, a capacitance part, a drive transistor and a light-emitting element. The sampling transistor conducts in response to the control pulse supplied from the scan line to sample the video signal supplied from the signal line. The capacitance part holds an input potential dependent upon the sampled video signal. The drive transistor supplies an output current during a certain emission period according to the input potential held by the capacitance part. Typically the output current has dependence on the carrier mobility in the cannel region of the drive transistor and dependence on the threshold voltage of the drive transistor. The output current supplied from the drive transistor causes the light-emitting element to emit light with a luminance dependent upon the video signal.
The drive transistor receives at the gate thereof the input potential held by the capacitance part to flow the output current between the source and drain, to thereby apply a current to the light-emitting element. Typically the emission luminance of the light-emitting element is proportional to the applied current amount. In addition, the amount of the output current supplied from the drive transistor is controlled with the gate voltage, i.e., the input potential written to the capacitance part. The conventional pixel circuit changes the input voltage applied to the gate of the drive transistor according to the input video signal, to thereby control the amount of a current supplied to the light-emitting element.
The operating characteristic of the drive transistor is expressed by the following equation.Ids=(1/2)μ(W/L)Cox(Vgs−Vth)2  Equation 1
In Transistor Characteristic Equation 1, Ids denotes a drain current flowing between the source and drain. The drain current corresponds to the above-described output current supplied to the light-emitting element in the pixel circuit. Vgs denotes a voltage applied to the gate (gate-applied voltage) with reference to the potential of the source. The gate-applied voltage corresponds to the above-described input potential in the pixel circuit. Vth denotes the threshold voltage of the transistor. μ denotes the mobility in a semiconductor thin film constituting the channel of the transistor. In addition, W, L and Cox denote the channel width, channel length and gate capacitance, respectively. As is apparent from Transistor Characteristic Equation 1, when a thin film transistor operates in the saturated region, a gate voltage Vgs higher than the threshold voltage Vth turns on the transistor to flow the drain current Ids. In principle, a constant gate voltage Vgs invariably provides the same drain current Ids to the light-emitting element as indicated by Transistor Characteristic Equation 1. Therefore, supplying the same level input signal to all pixels of a screen should cause all the pixels to emit light with the same luminance, and thus should achieve the uniformity of the screen.
In fact, however, there are variations in device characteristics among TFTs formed of a semiconductor thin film, such as a poly-silicon thin film. In particular, the threshold voltage Vth is not constant but varies from pixel to pixel. As is apparent from Transistor Characteristic Equation 1, even if the gate-applied voltage Vgs is constant, variation in the threshold voltage Vth among the drive transistors leads to variation in the drain current Ids. Thus, the luminance varies depending on each pixel, which spoils the uniformity of the screen. Conventionally, there has been developed a pixel circuit that has a function of canceling variation in the threshold voltage among drive transistors. For example, such a pixel circuit is disclosed in the above-mentioned Japanese Patent Laid-Open No. 2004-133240.
The pixel circuit provided with the function of canceling variation in the threshold voltage can improve the uniformity of a screen to some extent. However, of the characteristics of poly-silicon TFTs, not only the threshold voltage but also the mobility μ vary depending on each element. As Transistor Characteristic Equation 1 shows, variation in the mobility μ results in variation in the drain current Ids even if the gate-applied voltage Vgs is constant. As a result, the emission luminance varies from pixel to pixel, problematically spoiling the uniformity of a screen.